1. Field of the invention:
This invention relates to a method for the growth of silicon carbide single crystals substantially free of defects such as stacking faults on a silicon single-crystal substrate.
2. Description of the prior art:
Silicon carbide (SiC) is a semiconductor material with a wide energy gap of 2.2 to 3.3 electron-volts (eV) as compared with conventional semiconductor materials such as silicon (Si) and gallium arsenide (GaAs), which have come into extensive use. Silicon carbide is thermally, chemically and mechanically quite stable, and also has a great resistance to radiation damage. The saturation drift velocity of electrons in silicon carbide is greater than that in silicon and other semiconductor materials. Moreover, silicon carbide has the advantage of having satisfactory stability in either case of p-type or n-type, which is rare for wide-gap semiconductors. In particular, this advantage makes it useful as a semiconductor material for optoelectronic devices utilizing visible light of short wavelengths. The use of semiconductor devices using conventional semiconductor materials such as silicon is difficult under severe conditions of high temperature, high output drive, high frequency operation, and radiation exposure. Therefore, semiconductor devices using silicon carbide are expected to have wide applications for devices which can be used under such conditions.
Despite these many advantages and capabilities, silicon carbide has not yet been placed in actual use, because the technique for growing silicon carbide single crystals with high reproducibility, which is required for the commercial production of high-quality silicon carbide wafers having a large surface area, has yet to be developed.
Conventional processes for preparing single-crystal substrates of silicon carbide on a laboratory scale include the so-called sublimation method (i.e., the Lely method) wherein silicon carbide powder is sublimed in a graphite crucible at 2,200.degree. C. to 2,600.degree. C. and recrystallized to obtain a silicon carbide single crystal, and the so-called epitaxial growth method wherein the silicon carbide single crystal obtained by the sublimation method is used as a substrate and silicon carbide single-crystal layers are then grown on the substrate by chemical vapor deposition (CVD) or liquid phase epitaxy (LPE), resulting in silicon carbide single crystals, the size of which is sufficient to produce semiconductor devices therefrom. Although a large number of crystals can be obtained by either the sublimation method or the epitaxial growth method, it is difficult to prepare large single crystals of silicon carbide and to control with high accuracy, the size and shape of silicon carbide single crystals. Moreover, it is also difficult to control the polytype and impurity concentration of these single crystals.
In recent years, the inventors have developed a process for growing large-sized high-quality single crystals of .beta.-silicon carbide on a silicon single-crystal substrate by the chemical vapor deposition (CVD) technique and filed a Japanese Patent Application No. 58-76842 (76842/1983) which corresponds to U.S. Pat. No. 4,623,425. This process (referred to as a successive two step CVD technique) includes growing a silicon carbide thin film on a silicon substrate by the CVD technique at a low temperature and then growing a silicon carbide single-crystal film on this thin film by the CVD technique at a higher temperature. Results for the application of this process have also been reported in the Journal of Crystal Growth, 70, 1984.
Also, another process for growing large-sized single crystals of .beta.-silicon carbide by the carbonization CVD technique is disclosed in Applied Physics Letters, 42(5), Mar. 1, 1983. This process includes heating the surface of a silicon single-crystal substrate in an atmosphere containing hydrocarbon gases to form a silicon carbide thin film thereon by carbonization and then growing a silicon carbide single-crystal layer on this thin film by the CVD technique.
Moreover, the inventors have devised a process for growing single crystals of .alpha.-silicon carbide and filed a Japanese Patent Application No. 58-246512 (246512/1983) which corresponds to U.S. patent application Ser. No. 683,651.
At the present time, these techniques make it possible to produce large-size high-quality single crystals or .alpha.- and .beta.-silicon carbide, while controlling the characteristics of crystals, such as polytype, impurity concentration, size and shape. These techniques are referred to as hetero-epitaxial growth methods in association with the growth of single-crystal layers on a single-crystal substrate which is made of a different material from that of the single-crystal layers.
In general, however, when such a hetero-epitaxial growth method is employed to form an epitaxially grown layer on a single-crystal substrate, the epitaxially grown layer has a tendency to contain crystal defects, inter alia, stacking faults, because there is a difference in lattice constant, coefficient of thermal expansion, and chemical bonding between the epitaxially grown layer and the single-crystal substrate.
The lattice constant of silicon single crystals is different from that of silicon carbide single crystals by as much as 20%, and hence, there may be many stacking faults generated on the {111} planes within the silicon carbide single crystals grown on the silicon single-crystal substrate. These stacking faults exert an adverse effect on the electronic properties of the silicon carbide single crystals obtained, thereby making it difficult to obtain silicon carbide semiconductor devices with excellent characteristics. Moreover, silicon carbide single crystals have a tendency to contain crystal defects referred to as antiphase boundaries, thereby making it difficult to produce silicon carbide semiconductor devices at desired positions on a silicon substrate.
Thus, none of the growth methods set forth above can provide silicon carbide single crystals substantially free of crystal defects such as stacking faults with high reproducibility. Therefore, a continuing need exists to establish a method for the production of silicon carbide single crystals with excellent crystallinity on an industrial scale.